A prior art yarn storing device of the above-mentioned kind is known from CH-C No. 62 54 84. The sensor of the yarn storing device arranged at the withdrawal end of the storage drum consists of an outer electrode in the form of a sleeve, an inner electrode and an isolating guiding body for guiding the yarn. During the running of the yarn along the isolating guiding body in its circumferential direction periodic noise signals are generated. In other words, the sliding movement of the yarn along the guiding body between the electrodes causes the generation of high frequent electric signals which are like an amplitude-modulated noise. This sensor is, due to its capacitive nature, very sensitive to any noise signals stemming from the surrounding, so that the electric signal indicating the movement of the yarn is superposed by a relatively high noise level. Hence, the detection of yarn movement by means of this sensor has turned out to be unreliable. Moreover, the signal generated by the sensor of this prior art yarn storing device depends on the particular properties of the yarn so that the necessary pre-setting of a detection circuit connected to the sensor of this prior art yarn storing device has to be adjusted when changing the type of yarn. For this reason, yarn storing devices which are commonly used in weaving machines are usually equipped with an electooptic sensor located close to the circumferential surface of the storage drum so as to detect a passing of the yarn through its detection area when withdrawing it from the drum.
Another type of yarn sensor is known from CH-C No. 44 00 73. The prior art yarn sensor described in this reference utilizes a piezoelectric detector element for detecting vibrations of a sleeve-like detector unit when the yarn is running longitudinally therethrough. When the yarn is moving through the detector element, a noise-like signal is generated by the piezoelectric element. This noise-like signal is caused by the friction of the yarn with respect to the detector element. The detector is not necessarily a piezoelectric one, but can also be an inductive detector, a magnetostrictive detector or a hall generator. These prior art sensors cannot only be used for detecting a movement of the yarn in its actual direction, but can also be used for detecting the changing motion of the running yarn at the yarn guiding drums of a spooling machine.
In view of this state of the art, the present invention is based on the object of achieving a yarn storing device of the above-mentioned kind having a more reliable detection of the withdrawal of the yarn from the storage drum.